Six years of observations by the European Space Agency’s (ESA) Venus Express have shown large changes in the sulfur dioxide content of the planet’s atmosphere, and one intriguing possible explanation is volcanic eruptions.

The thick atmosphere of Venus contains more than a million times as much sulfur dioxide as Earth’s, where almost all of the pungent toxic gas is generated by volcanic activity.

Most of the sulfur dioxide on Venus is hidden below the planet’s dense upper cloud deck because the gas is readily destroyed by sunlight. That means any sulfur dioxide detected in Venus’ upper atmosphere above the cloud deck must have been recently supplied from below.

Venus is covered in hundreds of volcanoes, but whether they remain active today is much debated, providing an important scientific goal for Venus Express. The mission already has found clues pointing to volcanism on geologically recent timescales within the past few hundreds of thousands to millions of years.

A previous analysis of infrared radiation from the surface pointed to lava flows atop a volcano with a composition distinct from those of their surroundings, suggesting that the volcano had erupted in the planet’s recent past.

Now, an analysis of sulfur dioxide concentration in the upper atmosphere over six years provides another clue.

Immediately after arriving at Venus in 2006, the spacecraft recorded a significant increase in the average density of sulfur dioxide in the upper atmosphere, followed by a sharp decrease to values roughly 10 times lower by today.A similar fall was also seen during NASA’s Pioneer Venus mission, which orbited the planet from 1978 to 1992. At that time, the preferred explanation was an earlier injection of sulfur dioxide from one or more volcanoes, with Pioneer Venus arriving in time for the decline.

“If you see a sulfur dioxide increase in the upper atmosphere, you know that something has brought it up recently because individual molecules are destroyed there by sunlight after just a couple of days,” said Emmanuel Marcq of Laboratory Atmospheres, Mediums, Space Observations in France.

“A volcanic eruption could act like a piston to blast sulfur dioxide up to these levels, but peculiarities in the circulation of the planet that we don’t yet fully understand could also mix the gas to reproduce the same result,” said Jean-Loup Bertaux, principal investigator for the instrument on Venus Express that made the detections.

Venus has a super-rotating atmosphere that whips around the planet in just four Earth days, much faster than the 243 days the planet takes to complete one rotation about its axis. Such rapid atmospheric circulation spreads the sulfur dioxide around, making it difficult to isolate any individual points of origin for the gas.

Marcq’s team speculate that if volcanism was responsible for the initial increase, then it could come from a relatively gentle increased output of several active volcanoes rather than one dramatic eruption. “Alternatively, and taking into account the similar trend observed by Pioneer Venus, it’s possible that we are seeing decadal-scale variability in the circulation of the atmosphere, which is turning out to be even more complex than we could ever have imagined,” Marcq said.

“By following clues left by trace gases in the atmosphere, we are uncovering the way Venus works, which could point us to the smoking gun of active volcanism,” said Håkan Svedhem from ESA.

Artist’s impression of an active volcano on Venus. Results from a long-term study of Venus find evidence of a clear injection of sulfur dioxide into its upper atmosphere. One possible interpretation is that volcanic activity increased the sulfur dioxide component of the upper atmosphere, although an alternative is that a change in atmospheric circulation dredged up the gas. // Credits: ESA/AOES

Six years of observations by the European Space Agency’s (ESA) Venus Express have shown large changes in the sulfur dioxide content of the planet’s atmosphere, and one intriguing possible explanation is volcanic eruptions.

The thick atmosphere of Venus contains more than a million times as much sulfur dioxide as Earth’s, where almost all of the pungent toxic gas is generated by volcanic activity.

Most of the sulfur dioxide on Venus is hidden below the planet’s dense upper cloud deck because the gas is readily destroyed by sunlight. That means any sulfur dioxide detected in Venus’ upper atmosphere above the cloud deck must have been recently supplied from below.

Venus is covered in hundreds of volcanoes, but whether they remain active today is much debated, providing an important scientific goal for Venus Express. The mission already has found clues pointing to volcanism on geologically recent timescales within the past few hundreds of thousands to millions of years.

A previous analysis of infrared radiation from the surface pointed to lava flows atop a volcano with a composition distinct from those of their surroundings, suggesting that the volcano had erupted in the planet’s recent past.

Now, an analysis of sulfur dioxide concentration in the upper atmosphere over six years provides another clue.

Immediately after arriving at Venus in 2006, the spacecraft recorded a significant increase in the average density of sulfur dioxide in the upper atmosphere, followed by a sharp decrease to values roughly 10 times lower by today.A similar fall was also seen during NASA’s Pioneer Venus mission, which orbited the planet from 1978 to 1992. At that time, the preferred explanation was an earlier injection of sulfur dioxide from one or more volcanoes, with Pioneer Venus arriving in time for the decline.

“If you see a sulfur dioxide increase in the upper atmosphere, you know that something has brought it up recently because individual molecules are destroyed there by sunlight after just a couple of days,” said Emmanuel Marcq of Laboratory Atmospheres, Mediums, Space Observations in France.

“A volcanic eruption could act like a piston to blast sulfur dioxide up to these levels, but peculiarities in the circulation of the planet that we don’t yet fully understand could also mix the gas to reproduce the same result,” said Jean-Loup Bertaux, principal investigator for the instrument on Venus Express that made the detections.

Venus has a super-rotating atmosphere that whips around the planet in just four Earth days, much faster than the 243 days the planet takes to complete one rotation about its axis. Such rapid atmospheric circulation spreads the sulfur dioxide around, making it difficult to isolate any individual points of origin for the gas.

Marcq’s team speculate that if volcanism was responsible for the initial increase, then it could come from a relatively gentle increased output of several active volcanoes rather than one dramatic eruption. “Alternatively, and taking into account the similar trend observed by Pioneer Venus, it’s possible that we are seeing decadal-scale variability in the circulation of the atmosphere, which is turning out to be even more complex than we could ever have imagined,” Marcq said.

“By following clues left by trace gases in the atmosphere, we are uncovering the way Venus works, which could point us to the smoking gun of active volcanism,” said Håkan Svedhem from ESA.